Considerations of "thickness" of film plane and sensitivity to lack of flatness etc.

Considerations of "thickness" of film plane and sensitivity to lack of flatness etc.

Having done the calculations for the DOF of the subject, what's the DOF of the image plane? How accurate must one be?

Let's say, for example using a 750 mm lens at f22 and 1.5 meters to the subject, what's the considerations that occur as to the thickness of the resultant image on a plane "in focus" 1.5 meters from the back of the lens?

Does it make much difference what thickness the paper is one uses to focus the lens before putting in the film? How much leeway is there? Our target focal plane is 50 inches wide by 74 inches high and made parallel to the taking lens plane using Zig Align. Is that instrument likely to be sensitive and accurate enough for keeping the planes parallel and how much offset can be allowed before the image is no longer sharp?

Asher

Hi, Asher,

Asher Kelman said:

Having done the calculations for the DOF of the subject, what's the DOF of the image plane? How accurate must one be?

Click to expand...

Your question is about the depth of focus. This is the range of locations of the film plane (measured from its ideal position, at which the subject is in perfect focus) over which the blurring due to misfocus does not result in a point on the subject being imaged as a circle of confusion whose diameter is greater than our arbitrarily-chosen COCDL.

That is given by the following equation:


where d is the permitted variation of the film plane location on either side of the perfect plane (it is symmetrical), m is the magnification, c is the chosen COCDL, and N is the f number of the lens.

Let's say, for example using a 750 mm lens at f22 and 1.5 meters to the subject, what's the considerations that occur as to the thickness of the resultant image on a plane 1.5 meters from the back of the lens?

Click to expand...

Suppose we choose as our COCDL 1/1400 the diagonal size of the image. For your image dimensions, 50" x 74" (a diagonal dimension of 89.3"'), we would thus choose c to be 1.6 mm.

Then from the equation above, with a magnification, m, of 1.0 (as is your case) and N=22 (f/22), d would be ±70 mm (±2.75").

Does it make much difference what thickness the paper is one uses to focus the lens before putting in the film?

Click to expand...

Not hardly.

Best regards,

Doug

Thanks so much, Doug!

Now what is the basis for choosing a COCDL of 1/1400 the diagonal of the image?

Asher

Hi, Asher,

Asher Kelman said:

Now what is the basis for choosing a COCDL of 1/1400 the diagonal of the image?

Click to expand...

That is a widely-used "classical" criterion. It is based on:

• Assuming that a viewer views the image at a distance approximately equal to its diagonal dimension.

• It assumes that a blur circle whose diameter (viewed in that way) subtends about twice the human angular acuity represents the worst "allowable" blurring.

That is of course a very arbitrary model.

We can of course adopt many other "models". We can assume that the viewer will view the image from some certain distance (not predicated on the image size), and make some other choice insofar as how big a blur circle (perhaps another multiple of human visual acuity) we will consider our limit of "acceptable blurring".

For example., suppose we wish to contemplate viewing the finished image from a distance of 24" and wish the blur circle to subtend an angle no greater than the typical human visual acuity. That would then suggest a COCDL of 0.2 mm.

Then, the depth of focus in the setup you mention would be about ±8.8 mm (±0.35").

That COCDL would correspond to about twice the typical diameter of a human hair (100 µm) seen from a distance of 24".

Best regards,

Doug

Great, now how about looking from 5" or 10 inches at the detail.

This is a type of picture one walks right up to.

Asher

Hi, Asher,

Asher Kelman said:

Great, now how about looking from 5" or 10 inches at the detail.

This is a type of picture one walks right up to.

Click to expand...

Well, if we wanted the blurring to be not greater than human visual acuity when viewed from a distance of 8", this would suggest a COCDL of 0.067 mm, and thus to a depth of focus for the case described of ±3 mm.

Are we expecting flatness of field comparable to that from the lens?

Are we expecting all the features of our subject to lie within a zone of about 6 mm thickness (the double-sided depth of field for this setup, using that COCDL)?

Perhaps the subject is a poster of Jayne Mansfield.

Best regards,

Doug

Doug Kerr said:

Hi, Asher,



Well, if we wanted the blurring to be not greater than human visual acuity when viewed from a distance of 8", this would suggest a COCDL of 0.067 mm, and thus to a depth of focus for the case described of ±3 mm.

Are we expecting flatness of field comparable to that from the lens?

Are we expecting all the features of our subject to lie within a zone of about 6 mm thickness (the double-sided depth of field for this setup, using that COCDL)?

Perhaps the subject is a poster of Jayne Mansfield.

Best regards,

Doug

Click to expand...

Doug,

Yes, that's all the DOF I expect in the subject at the eyes of just ±3 mm. As long as that's fine, everything else will be what it will be. I can get a flatness of the plane of the vacuum board of ± 0.001" per square foot.

Hi, Asher,

Asher Kelman said:

Yes, that's all the DOF I expect in the subject at the eyes of just ±3 mm. As long as that's fine, everything else will be what it will be.

Click to expand...

Makes sense.

As to the flatness of field of the lens, if your concentration is just on the eyes, that should not be much of an issue either.

Best regards,

Doug

Well, Doug,

I know I can get the material flat enough by simply paying for it, option #1. But can I make it with pegboard to remain flat when vertical with the compartment behind it? Option #2.

Since the cost of home-built is 1/100 of the former, I resort to my older motto, (prior to "Good enough" isn't!) which is "Don't "think", try!"

Asher

These numbers seem to jive with my experience with 20"x24"...the classic Polaroid-made 20"x24" cameras use roll film pulled over an idler roller in front of the (somewhat flexible plastic) focusing screen(seemingly plenty of room for film displacement from focal plane)
In experience, if it was sharp on the GG, it's sharp on the film.
Best,
Tracy Storer
20x24 West

Thanks Tracy for your opinion.

It's especially valuable as you have so much experience using ULF cameras and also building them! I need to figure out a fine adjustment slider with a geared hand drive for the last 3 cm of adjustment.

Asher

Hi, Asher,

Asher Kelman said:

I know I can get the material flat enough by simply paying for it, option #1. But can I make it with pegboard to remain flat when vertical with the compartment behind it? Option #2.

Click to expand...

I would think that if you had support for the pegboard periodically (i.e., "columns" of 3/4" dowel running from the back of the "box" to the pegboard, perhaps on 12" centers) that would help a lot. They could be cut to length accurately with a chop saw using a stop block clamped to the fence.

You could glue the columns to the back of the "box". Locate them so they fall between the holes on the pegboard. Then fasten the pegboard to them with flat-head wood screws in countersunk holes .

Just a thought.

Best regards,

Doug

Doug Kerr said:

Hi, Asher,
You could glue the columns to the back of the "box". Locate them so they fall between the holes on the pegboard. Then fasten the pegboard to them with flat-head wood screws in countersunk holes .

Click to expand...

I was planning to use strips of wood or aluminum with holes at intervals to allow cross circulation of air. but am also looking at open air cell aluminum foam panels.

Asher

Hi, Asher,

Asher Kelman said:

I was planning to use strips of wood or aluminum with holes at intervals to allow cross circulation of air. but am also looking at open air cell aluminum foam panels.

Click to expand...

All sounds good.

Never heard of that!

This should be a nice project.

Best regards,

Doug

I have just posted here:

http://www.openphotographyforums.com/forums/showthread.php?p=119701#post119701

a derivation of the equation for depth of focus, as cited earlier in this thread.

Best regards,

Doug